The present invention relates to a recording medium excellent in ink absorptivity which is suitable for use as a recording medium for an ink jet printer, and a process for its production, and a process for producing a silica-alumina composite sol.
In recent years, along with widespread use of digital cameras or computers, hard copy technology to record such images on paper, etc. has been rapidly developed. As recording systems for hard copies, not only one which directly photographs a display showing the image by silver salt photography, but also various other systems such as a sublimation thermal transfer system, an ink jet system, etc., have been known.
Among them, the ink jet system is one wherein ink droplets comprising a solvent and a dye or pigment are jetted at a high velocity from a nozzle to a recording medium, and in the ink, a large amount of a solvent is contained. A printer employing such an ink jet system has found remarkable widespread use in recent years, since it is easy for full coloring or high speed modification, or its printing noise is low.
In particularly, a printer employing a photo ink of a dye capable of forming a printed image close to a photograph, has been marketed recently, and its market is rapidly expanding by virtue of the good image quality. For a printer of this type, a large amount of thin ink is used for expression of a color of intermediate gradation, whereby the amount of ink tends to be large as compared with a case where a conventional ink is employed. When the amount of ink increases, unless the recording medium readily adsorbs the ink, the ink tends to be flooded and run, or absorption irregularity of ink so-called beading tends to result, whereby the commercial value will be impaired. Therefore, a recording medium for a photo ink jet printer is required to absorb a large amount of ink more quickly than ever.
Further, an ink jet printer employing a pigment ink is expected to rapidly gain popularity in future, since it has a feature that as compared with an ink jet printer employing a dye ink, the weather resistance of printed images is excellent. Also such an ink jet printer employing a pigment ink, is required to absorb a large amount of ink quickly from the view point of high image quality and high speed printing.
Heretofore, as a recording medium for an ink jet printer, one comprising a substrate such as a film or paper sheet and a porous ink-receiving layer formed thereon, which is composed of inorganic fine particles of e.g. silica or alumina, and a binder such as polyvinyl alcohol, has been known. For example, JP-A-2-276670 discloses a recording medium comprising a substrate and an ink-receiving layer formed thereon, which is composed of pseudo boehmite. Further, JP-A-10-231120 discloses a powder of alumina hydrate and alumina sol which has a large pore volume and a large average pore radius and is excellent in ink absorptivity and transparency, to be used for an ink-absorbing layer of a recording medium.
Further, the present inventors have previously proposed in WO99/64354 a recording medium provided with a porous layer having good ink absorptivity, prepared from a coating fluid employing a certain specific silica-alumina composite sol.
However, as mentioned above, with an ink jet printer employing a dye photo ink or a pigment ink, a high image quality and high printing speed will be required more than ever, and accordingly, as an ink-receiving layer, it is required to absorb the solvent contained in a large amount in the ink as quickly as possible. Thus, a recording medium having an ink-receiving layer having a higher ink absorbing ability than ever, has been desired.
A silica type material such as silica gel has proper pores, but, usually, the surface of silica particles is negatively charged and can not adsorb a direct dye or an acid dye having anionic dissociative groups, which is employed for an ink jet system, and the water resistance is low.
Therefore, JP-A-60-257286 discloses a method wherein poly aluminum chloride is incorporated to an ink-receiving layer. However, the poly aluminum chloride is a water-soluble salt. Accordingly, the poly aluminum chloride in the ink-receiving layer is likely to be dissolved in water and cause pit-like defects on appearance on the surface of the ink-receiving layer, and the water resistance has not yet been necessarily adequate. Further, when the medium is stored for a long period of time, the poly aluminum chloride is likely to migrate and clog pores in the ink-receiving layer, whereby the ink absorptivity tends to deteriorate.
Further, JP-B-47-26959 discloses a method for producing a positively charged colloidal silica sol, which comprises coating the silica surface with alumina. Namely, a silica sol having a particle diameter of from 2 to 150 nm is gradually added to an aqueous solution of poly aluminum chloride, and this mixture is aged until the pH becomes constant, i.e. usually until the pH becomes not higher than 4, followed by an addition of an alkali to bring the pH of the mixture to a level of from about 4.5 to 7.0. By this method, it is possible to obtain a silica sol having the surface coated with alumina, which is excellent in transparency and stability. However, the sol is not in the form of secondary aggregate particles, and the pore volume and the pore radius of a xerogel obtained by drying the sol, are small, and the ink-receiving layer formed by using it tends to have inadequate ink absorptivity.
An ink-receiving layer formed by using an alumina hydrate such as pseudo boehmite, is excellent from the viewpoint of the ink absorptivity, transparency, water resistance, gloss, etc. However, it has had a problem from the viewpoint of the scratch resistance. This is believed to be attributable to the fact that the aluminum hydrate is not spherical. To solve this problem, JP-A-7-76162 discloses a method wherein a silica gel layer having a thickness of from 0.1 to 30 xcexcm is formed on a porous layer made of pseudo boehmite. However, this silica gel layer has a drawback that it impairs the ink absorptivity, and from the viewpoint of industrial production, such a double layer structure is disadvantageous.
The present invention provides a process for producing a silica-alumina composite sol, which comprises mixing a silica hydrogel and an aluminum salt which, when dissolved in water, gives an acidic solution.
Further, the present invention provides a process for producing a recording medium, which comprises coating on a substrate a coating fluid comprising a binder and a silica-alumina composite sol obtained by mixing to a silica hydrogel an aluminum salt which, when dissolved in water, gives an acidic solution, followed by drying to form an ink-receiving layer.
Still further, the present invention provides a recording medium comprising a substrate and an ink-receiving layer formed thereon, wherein in the ink-receiving layer, the total pore volume of pores with radii of from 0.7 to 100 nm as measured by a nitrogen adsorption/desorption method, is at least 0.8 cm3/g, and the average pore radius as defined by a radius corresponding to a pore volume of xc2xd of the pore volume of pores with radii of from 0.7 to 100 nm in a cumulative pore volume distribution curve obtained by a BJH method from a nitrogen desorption isothermal line, is at least 14 nm.
Furthermore, the present invention provides an ink jet recording medium comprising a substrate and a porous ink-receiving layer formed thereon, whereby when a black ink drop having a diameter of 50 xcexcm and a volume of 65 pL is printed on the surface of the ink-receiving layer of the recording medium, the time for absorption of the ink drop is at most 10 msec, and the dot diameter obtained from the geometric average after the printing is at most 115 xcexcm.
According to the present invention, as compared with conventional recording media, a remarkable large ink absorptivity can be attained without deteriorating the properties such as color-reproducibility, image density and weather resistance. This is attributable to the fact that the ink-receiving layer of the recording medium of the present invention has a pore volume and an average pore radius which are so large as have not been seen before. It has been found possible for the first time by the present inventors that such large pore volume and average pore radius can be accomplished by a recording medium having an ink-receiving layer containing a novel xerogel having certain specific physical properties, which is obtainable from the above-mentioned specific silica-alumina composite sol.
Accordingly, such large pore volume and average pore radius can not be accomplished with a xerogel obtainable from a composite sol as described above and as shown in WO99/64354, even if the same silica-alumina composite sol is used.